A torsion spring is an elastic element capable of being flexed by twisting about its axis. The spring action, or spring constant, is primarily a result of the relationships between the torque applied in twisting the spring, the angle through which the torsion spring twists, and the modulus of elasticity of the spring material in shear. The twisting action on a torsion spring is produced by couples, that is, two equal forces producing partial rotation by moving in parallel but opposite directions and acting normal to the axis of the shaft.
As we understand the state of the art, torsion springs employed industrially have been strips, rods, or bars, generally termed shafts, usually with square, elliptical, rectangular, circular, or triangular cross-sections. As such they are used in machines, passenger cars and trucks. Tubular members find application in aircraft and the like. Shear stresses are assumed uniform over the tube wall thickness. Cylindrical bars can be visualized as helical springs with a resisting torque equal to applied torque. When non circular shafts are twisted, their sections become warped and stresses do not vary linearly as in the case of shafts having circular cross-sections.
Torsion springs have been used as springs for years, generally in the form of torsion bars. There is, then, a need for a different type of torsion spring, especially for such uses as scanning devices which are extensively employed in space research. In addition to their use in scanning the earth and other planets, x-ray, gamma-ray, and similar scanning instruments perform important functions in space exploration, As will become apparent the torsion springs of this invention lend themselves to such usage.
By another embodiment of this invention a variable stiffness torsion spring is provided. In other words a torsion spring can be made which can be so adjusted as to have a given spring constant. These torsion springs are extremely useful in gimballed payloads such as sensors, telescopes and electronic devices on such platforms as a space shuttle, a space station, on experimental balloons, and on free-flying spacecraft. As will be seen, variable stiffness torsion springs are particularly useful in scientific instruments in which scan patterns are achieved through the use of rotating unbalanced masses. Such rotating unbalanced mass (RUM) devices are the subject of our U.S. Pat. No. 5,129,600 and our copending patent application Ser. No. 08/123,629 now U.S. Pat. No. 5,396,815 filed Sep. 15, 1993. As will be explained, the adjustable spring constant torsion springs provided herein eliminate the need for installing a different torsion spring in a scanning instrument when the scan frequency is changed.